1. Field of the Invention
This invention relates to tension connectors of the type having means to indicate that a pre-determined tension stress has been reached. It has particular applicability to anchor bolts installed in foundations, as well as to other bolts which cannot be readily removed for substitution and replacement by a bolt set for a different stress level.
2. Description of the Related Art
Anchor bolts are typically installed deep in concrete foundations, with threaded ends projecting upwardly for securement of vibrating machinery to the foundation. Their use presents problems not adequately met by bolts heretofore available.
To hold down vibrating machinery at a plurality of support points, it is important that the bolts be placed in tension at a level that is never fully relaxed regardless of the range of stress reversals imposed by vibrations. It is obviously desirable that such bolts be provided with some indicator means which on installation can be checked to determine that a desired pre-tension has then been applied. However when subjected to actual operation, the bolts may require further tightening; and new factors, such as vibration levels, may bring about the decision to change the preliminarily chosen stress levels. Bolts shown in prior patents do not fill this need.
All relevant patents employ the principle that steel is an elastic material; when stress not exceeding its elastic limit is applied, it elongates proportionately to the load, and resumes its precise original length when the tension is removed.
U.S. Pat. No. 4,525,114 to Rotabolt Limited, shows a tension bolt having in its headed end an axial cavity of relatively large diameter, which terminates inwardly in a constricted portion in which the inner end of a headed pin is positively anchored so that it cannot turn. The head of the pin normally projects slightly beyond the bolt head. Under the pin's head is a washer which is rotatable when the bolt has not been elastically extended in tension. As a design tension stress is reached in the bolt, the bolt cavity is elastically elongated, clamping the washer between the heads of the pin and the end of the bolt.
U.S. Pat. No. 4,636,120 to The Babcock & Wilcox Company shows substantially the same concept, using both a flat washer and a cupped washer. The same general idea is shown in U.S. Pat. No. 3,881,392 to Illinois Tool Works, Inc.
If a need is present to tighten a bolt to a tension level which is adjustable, the foregoing constructions will not serve. For this more complex purpose, a very complicated apparatus is shown in U.S. Pat. No. 4,686,859 to SPS Technologies. That patent similarly employs a pin securely fastened in the bottom of an axial cavity, whose outer end is headed by a slender disc. Imposing tension lessens the space between the disc and the top surface of the bolt. This lessening is measured by a complex gaging device to determine the tension stress in the bolt.
Another patent using an external gaging device is U.S. Pat. No. 3,954,004 to Orner. In this patent a large diameter cavity is filled with an incompressible substance such as liquid or rubber. As the cavity is lengthened in tension, the pin is drawn downward. An external gage is provided to measure the pin displacement.
All these patents show the cavities to be formed in the headed ends of bolts, reflecting the thinking that loss of the cross-sectional area to the cavity is not as serious as if the cavity were formed in a threaded end of the bolt.